The present invention relates to a method for handling mirror surface finished semiconductor wafers and an apparatus therefor.
In a typical technique used in the manufacture of semiconductor devices, a semiconductor layer of a specific conductivity type is formed on a semiconductor substrate by a CVD method and a functional element is formed in this region. In this CVD method, it is necessary to prevent an impurity from diffusing or mixing in from the substrate into the semiconductor layer.
Recently, a demand for an element with a high dielectric withstand voltage is high. Dielectric withstand voltage can be improved by thickening the semiconductor layer. However, it takes extremely long time to do this by the CVD method, and it is more difficult to prevent a diffusing of impurity.
In the invention described in Japanese patent publication No. 49-26455, semiconductor wafers are bonded such that semiconductor layers oppose each other, so as to prevent impurity mixing and to reduce manufacturing time as compared with the case wherein a semiconductor layer is formed using the CVD method. The method is performed as follows. Mirror surface finished semiconductor wafers are placed to oppose each other in an atmosphere of a gas mixture consisting of oxygen and nitrogen, and phosphorus is diffused into each mirror surface at a temperature of 1,200.degree. C. for 30 minutes using phosphorus oxychloride as an impurity. The wafers are then heated at a temperature of 1,150.degree. C. for an hour. After placing the wafers in a vacuum, the wafers are bonded at a pressure of 150 kg/cm.sup.2 and a temperature of 1,300.degree. C.
However, when a bonding temperature is higher than that during diffusion as described above, the diffusion impurity is redistributed during bonding to change a surface concentration of an impurity layer. In addition, wafer alignment in a vacuum is difficult to perform and results in high cost.
In order to solve the above problem, a method of bonding wafers in air at room temperature is disclosed by the present inventor in "Nikkei Electronics", Jan. 27, 1986, p. 108 to 110. In this method, after the mirror surfaces of wafers are washed with water and excess water is removed, the wafers are bonded together in a clean atmosphere utilizing residual water on the mirror surfaces. According to this method, impurity redistribution is prevented and wafer alignment is easy.
However, in this method, air bubbles are left between wafers to form voids in their interfaces.